An inlet valve is used to supply steam via a main high-pressure inlet or at a reheat intermediate pressure inlet to a steam turbine. Typically, such valves comprise a combined valve assembly incorporating two valve disks that perform the required functions in conjunction with a single valve seat. For example, in a main high-pressure steam inlet line, the upper valve disk is referred to as a control valve or intercept valve and the lower valve disk is referred to as a main stop valve or reheat stop valve, hereafter called control and stop valves, respectively. The control valve disk conventionally includes an annular valve element having a bottom opening recess, the margins of which valve disk seat against a valve seat in a closed position of the control valve. The control valve has a proportional control device, for example, a servo valve, for positioning the control valve disk relative to the valve seat to control the flow through the steam inlet.
The stop valve disk is typically controlled from below the inlet for movement between a fully valve-open position within the recess of the control valve and a valve-closed position seating on the steam inlet valve seat. Consequently, during operation, the stop valve is normally fully opened at all times and the control valve is opened to a greater or less extent to control the steam flow to the turbine section by throttling the control valve disk. For example, if the turbine speeds up above normal speed, the control valve starts to close rapidly to avoid turbine operation going to a destructive overspeed condition. The stop valve remains in its full open position. If the turbine speed continues to increase, a fast close signal is applied to both the stop valve and the control valve to close the steam inlet valve to the turbine. The stop valve closes immediately because it is a faster acting valve than the control valve.
When the control valve is throttled, typically below about 50% of maximum disk lift, flow phenomena within the valve may cause instabilities which, in turn, result in undesirable vibration, leading to component wear and/or fatigue-related failure of the valve or associate piping or turbine systems. Typically, control valves were not previously used in a tight control mode of operation, i.e., constant throttling at very low valve lift, high-pressure and low flow, so that the valve passageway is quite tight. Because the combined valves were not used in the past for extensive periods in a throttling mode, this inherent problem has not been addressed. However, recent valve designs have required more extensive throttling of the combined valves, causing vibration to become an issue due to excessive wear of the valve and/or associated piping components requiring maintenance or forced outages as a result of severe vibration.
After analysis, the cause of the problem appears to reside in the separation of the steam flow from walls defining the flow passage past the control valve and seat. Vortex flow is believed to occur downstream of the control valve and seat, causing a change in the forces on the control valve disk, in turn causing the disk to oscillate.